Current Perspectives on the Mechanisms of Chlorohydrocarbon Degradation in Subsurface Environments: Insight from Kinetics, Product Formation, Probe Molecules, and Isotope Fractionation

Degradation of chlorinated organic contaminants by natural and engineered reductive dechlorination reactions can occur via numerous biotic and abiotic transformation pathways giving rise to either benign or more toxic products. To assess whether dechlorination processes may lead to significant detoxification (a) the thermodynamic feasibility of a reaction, (b) rates of transformation, and (c) product formation routes need to be understood. To this end, fundamental knowledge of chlorohydrocarbon (CHC) reaction mechanisms is essential. We review insight from reaction thermodynamics, structure-reactivity relationships, and applications of radical and carbene traps, as well as of synthetic probe molecules. We summarize the state-of-knowledge about intermediates and reductive dechlorination pathways of vicinal and geminal haloalkanes, as well as of chlorinated ethenes. Transformation conditions are identified under which problematic products may be avoided. In an outlook, we discuss the potential of stable carbon and chlorine isotope fractionation to identify initial transformation mechanisms, competing transformation pathways, and common branching points.